In the production of silver halide photographic materials, a silver halide photographic emulsion is usually prepared by mixing a solution containing a silver ion with a solution containing a halide in a protective colloid by various methods to form fine silver halide grains, subjecting the resulting emulsion to sensitization by, for example, using a chemical sensitizer and, if desired, adding various photographic additives thereto. The thus-prepared silver halide emulsion is then coated on a support, followed by drying. A silver halide emulsion should, therefore, necessarily pass through a liquid state before being coated. Conditions for preserving the emulsion from the time of prepration until it is coated on the support (such as temperature, time, stirring conditions, etc.) influence certain photographic characteristics of the resulting photographic materials, such as sensitivity, gradation, fog, and the like. In fact, it is rare to obtain consistent photographic characteristics of silver halide photographic materials, i.e., it is difficult to always ensure high reproducibility in photographic characteristics even if the conditions for producing the silver halide photographic materials are as strictly controlled as possible to avoid fluctuations. In order to impart consistent photographic performance properties to silver halide photographic materials, it is essential to prepare a silver halide emulsion which exhibits excellent preservability during the period from its preparation through coating.
Known methods for improving the stability of silver halide emulsions during this period of preservation while the emulsion is in a liquid state include a method disclosed in Japanese Patent Application (OPI) No. 217928/83 (the term "OPI" as used herein refers to a "published unexamined Japanese Patent Application"), in which a mercaptoimidazole type compound and/or a triazole type compound is/are added to a silver halide photographic emulsion to improve its preservability; a method disclosed in Japanese Patent Application (OPI) No. 9658/84, in which a sensitizing dye is added to a silver halide emulsion after completion of chemical ripening and before coating to improve preservability of the emulsion; and a method disclosed in Japanese Patent Application (OPI) No. 29243/84, in which production stability of silver halide photographic materials can be improved by constructing the outer surface of silver halide grains with a {111} face and a {100} face at a given area ratio.
However, these methods do not improve liquid stage preservability of silver halide emulsions to a level satisfactory for practical usage. Further, even if the object of improving preservability to a desired level is achieved, secondary effects produced by these methods may interfere with the expected photographic characteristics of the resulting silver halide photographic materials. More specifically, it is possible to keep photographic characteristics of a silver halide emulsion consistent during the period from the initial emulsion preparation to coating by incorporating a mercaptoimidazole type compound and/or a triazole type compound into the silver halide emulsion as taught by Japanese Patent Application (OPI) No. 217928/83. However, addition of these compounds in amounts sufficient to produce such an effect on preservability hinders development of the resulting silver halide photographic materials, resulting in a reduction in sensitivity. Further, it is possible to maintain the photographic characteristics of a silver halide emulsion during the period from the initial emulsion preparation through the coating step by incorporating a sensitizing dye into the emulsion in an amount larger than is usually necessary for sensitization, as disclosed in Japanese Patent Application (OPI) No. 9658/84, but addition of a sensitizing dye in such an amount concomitantly causes a reduction in sensitivity in the inherently sensitive region. When this method is applied to color light-sensitive materials, the sensitizing dye present in the silver halide emulsion layer diffuses into other light-sensitive layers, ultimately resulting in unfavorable influences on photographic performances, such as a deterioration of color reproducibility. In addition, in those of the above-described techniques where an adsorbing substance is adsorbed onto the surfaces of silver halide grains in an emulsion, a resulting effect of maintaining the photographic characteristics consistent may occur. This effect occurs through the prevention of recrystallization of the silver halide grains due to a difference of individual grains in size or halogen composition in a liquid state. However, when the adsorbing substance is present in the liquid state silver halide emulsion, equilibrium is attained through repetitive absorption of this substance onto the silver halide grains and subsequent desorption therefrom. Therefore, even if the adsorbing substance, when viewed macroscopically, appears to be adsorbed on the silver halide grains, recrystallization of silver halide grains due to differences in size or halogen composition cannot be adequately inhibited. In these circumstances, the silver halide grains in a liquid state undergo changes with the passage of time, resulting in the photographic characteristics of the silver halide emulsion fluctuating over time. Furthermore, in the method disclosed in Japanese Patent Application (OPI) No. 29243/84, if the individual silver halide grains have a difference in halogen composition among themselves, a thermodynamic effect occurs such that the silver halide grains are dissolved whereby the composition of the grains becomes more uniform as to the halogen component. As a result, the silver halide grain composition in the emulsion changes. Therefore, this technique cannot also sufficiently inhibit fluctuations of the photographic characteristics of the emulsion in a liquid state with the passage of time.
Hence, all of the conventional techniques as mentioned above fail to sufficiently improve preservability of a silver halide emulsion in a liquid state because no technical improvements have been made in connection with the thermodynamic stability of the silver halide grains per se.